Due to the widespread popularity of the World Wide Web, internet traffic is at an all time high and rapidly increasing. The resulting congestion is taking its toll on users and telephone companies alike. Users are often frustrated by the length of time it takes to download complex graphics and videos. For example, a ten megabyte video clip which is the equivalent of a four minute movie, takes approximately ninety-three minutes using a 14.4 kilobyte modem and forty-six minutes using a 28.8 kilobyte modem.
In addition, lengthy data transmissions are tying up telephone company switches that were designed to handle brief telephone calls. Broadband modems, and in particular asymmetric digital subscriber line (ADSL) modems are an emerging technology which promises to dramatically increase the ability to transfer data over conventional telephone lines. Significantly, ADSL modems allow data transfers at rates over two hundred times faster than current modems, and over ninety times faster than ISDN lines.
ADSL was originally conceived of as a technology for delivering interactive multimedia services, such as video on demand over existing telephone networks. However, it is internet access that is currently driving the demand for ADSL. Unlike other high speed data transmission technologies such as ISDN, ADSL requires no massive rewiring or other changes to a telephone company's local exchange or central office. Notably, ADSL modems use the existing telephone infrastructure, including the so-called "last mile" of the network, which is the leg from the central office to a subscriber site (e.g., a home or office) which uses a twisted pair of copper lines. Although it is often referred to as the "last mile", the leg from the central office to the subscriber site is typically approximately 12,000-18,000 feet long.
The bandwidth of a conventional copper twisted pair telephone line is approximately 1 MHz. However, conventional analog signals which carry voice over these lines operate in a bandwidth which is only 4 KHz wide. Advantageously, ADSL takes advantage of the remaining portion of the 1 MHz. Specifically, ADSL technology effectively subdivides the 1 MHz bandwidth of the copper twisted pair line into three information channels: i) a high speed down stream channel, ii) a medium speed duplex (upstream/downstream) channel, and iii) a conventional voice channel. Downstream refers to transmissions from the telephone network to the ADSL modem located at a subscriber site, while upstream is the route from the subscriber site to the telephone network. This multichannel approach enables subscribers to access the internet, order a video for viewing and send a facsimile or talk on the telephone all at the same time.
FIG. 1 illustrates a communication system 10 which employs ADSL technology. The system 10 includes a subscriber site 12 which includes a phone 14, a facsimile machine 16 and a personal computer or computer network 18. The subscriber site 12 receives a twisted pair of copper telephone lines 20 which connect the subscriber site with a telephone central office 22. The run length of the telephone line 20 between the subscriber site and the central office 22 is typically between 12,000 and 18,000 feet. A POTS splitter 24 located at the subscriber site 12 is connected to the telephone line 20 and couples the telephone line to an ADSL modem 26 and to the phone 14 and facsimile machine 16.
Central office 22 includes a POTS splitter 30 which is operatively connected to an ADSL modem rack 32 and to a public telephone switch 34. As known, the public telephone switch 34 communicates over a public switch telephone network 36. The ADSL modem rack 32 also communicates over the public switch telephone network and is connected via an internet backbone 38 to various devices including a video server 40, a video conferencing server 42 and a World Wide Web server 44.
FIG. 2 is a functional block diagram illustration of the ADSL modem 26 and the POTs splitter 24. The modem 26 includes a hybrid circuit 50 which couples a transmit circuit 52 and a receive circuit 54 to the telephone line 20.
The transmit circuit 52 includes a digital signal processor (DSP) 56 which provides a digitized transmit signal on a line 58 to a digital-to-analog converter (DAC) 60. The resultant analog signal is input to a low pass filter (LPF) 62 and a filtered transmit signal is provided on a line 64 to the hybrid circuit 50.
The receive circuit 54 receives a signal on a line 66 and includes a high pass filter 68, a programmable gain amplifier 70, a low pass filter 72, an analog-to-digital converter (ADC) 74 and a DSP 76 which together process the signal on the line 66 in a known manner.
The POTs splitter 24 includes a high pass filter 78 and a LPF 80. The LPF 80 has a cut-off frequency set at approximately 4 KHz in order to allow the voice band signal to pass onto the line 28. A problem with this system architecture is that the HPF 78 filters signals which are being transmitted and received by the modem. Therefore, the cut-off frequency of the HPF 78 can be set at no higher than about 30 kHz to ensure that signals from the transmit circuit 52 pass relatively unattenuated through the POTs splitter. In addition, the hybrid 50 is typically used to terminate the HPF 78 in this embodiment.
A problem in prior art hybrids is that an excessive portion of the signal from the transmit circuit 52 undesirably couples to the receive circuit 54 through the hybrid 50 as noise. A measure of the amount of transmit signal power which couples to the receive signal is "transhybrid attenuation". If the transhybrid attenuation is too low, the signal-to-noise ratio (SNR) of the receive circuit decreases in part because the dynamic range of the ADC 74 in the receive circuit 54 decreases. The severity of this problem increases since the power of the transmit signal on the line 64 is much greater than the power of the receive signal on the line 66. Therefore, there is a need for an improved hybrid for coupling a broadband modem to a POTs line.